There are several ways to actually test driving ability. A standard driver’s test at the licensing bureau is one way to find out how “road worthy” a person may be. There are also special tests given to older adults who have had a stroke or other disabling injury. These are usually administered by an occupational therapist in a rehab center. Your mother could make arrangements to take either one of these tests.
There are some studies on pain and driving ability. A recent study reported that pain, pain meds, or the two combined together can increase the number of traffic accidents. People affected most often were middle-aged between 35 and 65 years old.
More accidents occur in older adults (over 65) who have other medical problems. Dementia, diabetes, arthritis, and heart disease are just a few examples. Women seem to be affected more often than men though it’s not clear why this is true.
It may be best to assess driving ability in city traffic and on the highway. You can start by asking your mother to drive you somewhere and see if you think there’s a problem. If talking to her is not helpful then contact her doctor and make the suggestion. She may be more willing to consider a test if her physician recommends it.